Reaction of S-2 with X/Mo(110) (X=Fe, Pt or Al) surfaces: Admetal-promoted sulfidation of Mo and the behavior of hydrodesulfurization catalysts

Exposure of a clean Mo(110) surface to S-2 at room temperature or at 700 K results in a chemisorbed layer of sulfur only, without any evidence for the formation of molybdenum sulfides. The effects of Fe, Pt, and Al on the reactivity of Mo(110) toward sulfur have been studied using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). It is found that at room temperature, the reactivity of S, with multilayers of the admetal follows the order Fe > Al >> Pt. While most of the Fe becomes sulfidized, only a few layers of the Al become sulfidized before passivation, and the least reactive Pt surface is covered by a chemisorbed layer of sulfur. Upon exposure of the Fe/Mo(110) (theta(Fe) <2 ML) surfaces to S-2 at elevated temperatures (700 K) or annealing of FeSy/Mo(110) surfaces (700-1000 K), the formation of MoSx compounds is observed, with FeSy remaining on the surface. High-temperature S-2 exposure of the Pt/Mol(110) system leads to the formation of a small amount of MoS, only in the case where theta(Pt)<1, with only a chemisorbed layer of sulfur observed on top the Pt surface in all other cases. There is no evidence for PtSx formation. High-temperature S-2 exposure of Al/Mo(110) or annealing of the AlSx/Al/Mo(110) system does not lead to formation of molybdenum sulfides. The thin layer AlSx film remains stable to further S, exposure. The effects of Fe, Pt, and Al on the rate of sulfidation of Mo are rationalized by examining the relative stabilities of the metal sulfides, the cohesive energies of the metals involved and electronic interactions in bimetallic bonding. The trends in our results are compared with trends in the activity of Mo-based bimetallic catalysts in hydrodesulfurization processes.